Department of Meteorology, Pennsylvania
State University, University Park, PA

1. INTRODUCTION

On the evening of 15 September 1999, Hurricane
Floyd made landfall at Bald Head Island, NC; 0630 UTC (hereafter all times
UTC) 16 September. Floyd was a large and intense Cape Verde hurricane
that affected the central and northern Bahamas before making landfall
on the North Carolina coast. At its peak intensity, Floyd reached the
top end of category four (with winds >69 m s-1) on the Saffir/Simpson
(Simpson 1974) hurricane scale, and arrived onshore as a category two
hurricane. Prior to and during the time of landfall the hurricane interacted
with a coastal front (Bosart et al. 1972; Bosart 1975; Keeter et al. 1995;
Garner 1999) and remained coupled with the front as the hurricane weakened
rapidly, and accelerated northward along the East Coast into New England
(Bosart and Atallah 2000; Kong 2000) (Fig. 1).

Eighteen tornadoes associated with Floyd
were reported in the U.S. and all occurred in North Carolina on the day
of landfall (Storm Data 1999). The two strongest tornadoes produced
F2 damage. Smith (1965) and McCaul (1991) identified the right-forward
quadrant of a hurricane as the most favored area for tornado development;
maximum shear and helicity values occur within the quadrant. Tornadic
events associated with Hurricane Floyd supported this assessment. What
is interesting about the landfall event was that 88.9% (16 of 18) of the
tornadoes developed either immediately along the coastal front or within
the warm sector east of the front.

Fig. 1. Path and progression
of Floyd along the East Coast (DDHH). Key locations mentioned in text are
labled.

National Weather Service rawinsonde data,
and NOAA/AOML Hurricane Research Division (HRD) GPS dropsonde data, were
used to create constant pressure and cross-sectional plots. These plots
were then subjectively analyzed for synoptic and mesoscale features. Soundings
from Morehead City, NC (MHX), Greensboro, NC (GSO), and Roanoke, VA (RNK),
launched at 0000 and 0600 on 16 September, were selected for frontal analysis.
These sites were downstream of the hurricane and closest spatially and
temporally to landfall location.

Weather Surveillance Radar-1988 Doppler
(WSR-88D) (Crum and Alberty 1993) level II data (Crum et al. 1993) from
MHX, Wilmington, NC (LTX), and Charleston, SC (CLX) were utilized to aid
in boundary placement. LTX was selected for analysis, as the radar was
located closest to the area of landfall and level II data were available
for the majority of the event, 1329 15 September to 1039 16 September,
inclusive. Unfortunately, level II data from CLX and MHX were either not
available or sporadic. Based on the work of Rasmussen et al. (2000), evidence
was presented that Storm Data reports must be verified for accuracy.
An effort was made to verify each report for location and time accuracy
utilizing WSR-88D level II data. Unfortunately, the lack of archived WSR-88D
data at MHX prevented a thorough investigation. Consequently, none of
the 18 tornado reports recorded by Storm Data could be refuted,
and all were assumed to be valid.

3. DATA AND RESULTS

A coastal front developed downstream
of the hurricane, during the mid-morning hours of 15 September, along
the coastal plain of the Carolinas and Virginia, 12 hours prior to
the landfall (Pietrycha 2001).

As Hurricane Floyd approached shore, the
pressure gradient over the coastal plain intensified, accelerating the
surface flow over the eastern Carolinas. The baroclinic zone associated
with the temperature gradient strengthened, owing to increased horizontal
deformation (Bluestein 1993), but remained nearly stationary throughout
the day. Frontogenesis would continue further north, immediately inland
from the shoreline and downstream of the advancing hurricane. By 0000
16 September, a coastal front oriented north-south was well established,
spanning the area east of Charleston, SC, to east of Virginia Beach, VA
(Fig. 2).

Fig.2. Surface map for 0000
UTC 16 September 1999, with subjectively analyzed mean sea level isobars
(solid) every 4 mb (labeled every other isobar to 988 mb), and isotherms
(dashed) every 2°C (labeled every isotherm). Standard station model
in abbreviated format; temperature in tenths °C and sky conditions reported.
Winds in knots with one pennant, one full barb, and one half barb equal
to 50, 10, and 5 knots, respectively.

Cross-sectional analysis of potential temperature
utilizing 0600 UTC rawinsonde data from RNK, GSO, and MHX, and surface
observations revealed a 1 km deep surface layer of cold air existed westward
from the coastal front to the mountains of North Carolina and Virginia
(Fig. 3). Warm air advection and strong isentropic
lift developed during the afternoon of 15 September over North Carolina,
downstream of the hurricane, in the deep layer flow normal to the boundary
and persisted as the storm moved onshore.

Concurrent with coastal front formation,
WSR-88D reflectivity imagery over eastern North Carolina displayed the
development of a persistent precipitation band in association with the
front (Fig. 4). Numerous rain bands were
evident in the data, rotating westward off the ocean and over North Carolina.
The spiral precipitation bands associated with Floyd maintained their
spatial continuity east of the front. As the spiral bands rotated westward
toward the coastal front, their discrete horizontal reflectivity structure
became absorbed by the more uniform frontal precipitation band. Stronger
convective cells (50-55 dBZ) embedded within the spiral bands were observed
to decrease in intensity and lose their reflectivity structure as the
spiral bands crossed the boundary over the cooler air.

Eighteen tornadoes occurred in North Carolina
on 15 September 1999 within the northeast quadrant of the hurricane (Fig.
5). All the parent supercells were associated with outer rain bands. No
tornadoes were reported in South Carolina or Virginia during the same
time frame. Of note is the fact that 88% (16 of 18) of tornadoes developed
either immediately on the coastal front or within the warm sector of the
boundary. The strongest two tornadoes resulted in F2 damage. One tornado
occurred 3 hours prior to the development of the front, within a quasi-homogeneous
airmass that existed downstream of the approaching hurricane, and another,
10 km on the cool side of the boundary. Both tornadoes produced F0 damage.

Fig. 5. Triangles denote the
locations of the 15 September 1999 tornadoes. Solid line represents the
mean location of the coastal front during the day/evening of 15 September
1999. Dashed line denotes the track of Hurricane Floyd.

The lack of tornadoes west of the boundary
is attributed to the inhibiting influence of the strong cold pool west
of the coastal front. In this case, the possibility for tornado genesis
appears to have been greatly diminished, given the elevated nature of
the storms along and over the 1 km deep cold pool. The same effects may
explain, in part, the absence of reported tornadoes in Virginia. With
the exception of extreme southeast Virginia, the state was blanketed by
cold air prior to and during hurricane landfall. McCaul (1991) has shown
that the left forward quadrant is least favorable for hurricane spawned
tornadoes; left front quadrant contains the least shear and helicity of
the four quadrants. Therefore, we can not refute the apparent lack of
tornadoes west of the boundary as a result of increasing spatial distance
into the left front quadrant of the hurricane.

As documented by Pietrycha et al. (2001)
the hurricane became secluded shortly after landfall; cold air advected
completely around the eye of the hurricane. The existence of the cold
airmass wrapped around the system may further explain the absence of reported
tornadoes within the left or right rear quadrants, as the hurricane departed
north of the region.

4. CONCLUSIONS

Eighteen tornadoes occurred over the state
of North Carolina prior to the landfall of Hurricane Floyd on 15 September
1999. During the day of landfall a coastal front developed downstream
of the hurricane. 88.9% of the reported tornadoes developed immediately
along and/or within the warm side of the boundary. The lack of tornadoes
west of the boundary was attributed to the inhibiting influence of the
strong cold pool west of the coastal front. In this case, the possibility
for tornado genesis appears to have been greatly diminished, given the
elevated nature of the storms along and over the cold pool.

A question arises; how deep is too deep
for a relatively stable boundary layer airmass to negatively impact tornado
genesis? Perhaps the answer will be resolved as our understanding of tornado
genesis advances. Nonetheless, forecasters are encouraged to be cognizant
of regions of deep cold air and/or hurricane/frontal boundary interactions
that might demise the threat of tornadoes and thus better define tornado
watch box coordinates and warnings.

ACKNOWLEDGMENTS

A sincere thank you to Mr. Michael Black
and Frank Marks (HRD) for providing the dropsonde data, Ms. Elke Edwards
for aiding in the preparation of this document, and helpful comments by
Drs. Paul Markowski (PSU) and Lance Bosart (SUNYA).